Processes for the polymerization of ethylene or mixtures of ethylene with alpha olefins to form copolymers having a density greater than 0.86 g/cm.sup.3 are known which are carried out in the gas phase in fluidized or mechanically stirred bed reactors, in the presence of catalysts obtained from compounds of transition metals belonging to groups IV, V or VI of the Periodic Table of the Elements and aluminum alkyl compounds or in the presence of catalysts based on chromium oxide.
The polymer is obtained in the form of granules having a more or less regular morphology depending on the morphology of the catalyst; the dimensions of the granules depend on the dimensions of the catalyst particles and on reaction conditions and are generally distributed around an average value.
In these types of processes the heat of reaction is removed by means of a heat exchanger placed inside the reactor or in the recycle line of the reaction gas.
A generally encountered problem in polymerization processes of this type results from the presence of very fine polymer particles which are either produced from already existing fine catalyst particles or derive as a result of breakage of the catalyst itself.
These fine particles tend to deposit onto and electrostatically adhere to the inner walls of the reactor and the heat exchanger, and to thereafter grow in size by chemical reaction thus causing an insulating effect and a lower heat transfer resulting in the formation of hot spots in the reactor.
These effects are enhanced when the gas-phase ethylene (co)polymerization process is carried out in the presence of highly active catalysts such as those comprising the reaction product of an aluminum alkyl with a titanium compound supported on a magnesium halide in active form.
As a consequence a loss in fluidization efficiency and homogeneity generally occurs; for example catalyst feeding interruption may occur as well as plugging of the polymer discharge system; furthermore, excess temperature can result in particle melting with the formation of fused polymer material/sheets which adhere to the reactor walls and may plug the gas distribution plate and/or the polymer discharge systems forcing shutdown of the reactor.
These drawbacks lead to poor process reproducibility and can lead to a forced interruption of the run in order to remove deposits which have formed inside the reactor even after relatively short times.
Several solutions have been proposed to avoid these drawbacks, either by acting on the catalyst activity or by reducing or eliminating the electrostatic voltage.
Patent Application EP-359444 describes the introduction into the polymerization reactor of small amounts (generally smaller than 0.1 ppm with respect to the polymerization mixture) of a retarder selected from polymerization inhibitors or substances able to poison the catalyst, in order to reduce the olefin polymerization rate. However, as described in the same patent application, the use of larger quantities of the retarder adversely affects both the quality and properties of the polymer produced, such as the melt index, the melt flow ratio and/or the stereoregularity of the polymer, as well as reducing the efficiency of the process.
U.S. Pat. No. 4,739,015 describes the use of oxygen containing gaseous products and liquid or solid compounds containing active hydrogens to prevent the formation of agglomerates and reactor fouling due to sticking of the elastomeric particles in processes for preparing heterophasic propylene polymers containing an elastomeric phase. Among the compounds containing active hydrogens, ethanol, methanol, ethylene glycol, propylene glycol and diethylene glycol are cited.
These compounds, which are known as polymerization inhibitors, must be used in an amount of a few ppm with respect to the polymer in order not to deactivate the catalyst; at such concentrations they are not effective as to a selective deactivation of the fine catalyst particles, whereas at higher concentrations the polymerization does not take place. Therefore, the use of the components described in said patent does not solve the problem of inhibiting the reactivity of the fine polymer particles present in a gas-phase (co)polymerization of ethylene and their consequent adhesion to the reactor walls.
Different techniques have been proposed to reduce or eliminate the electrostatic voltage responsible for the phenomena of migration and formation of deposits on the walls.
In U.S. Pat. No. 4,803,251 a group of chemical additives is described which generate both positive and negative charges in the reactor, and which are fed to the reactor in an amount of a few ppm per part of the monomer in order to prevent the formation of undesired positive or negative charges. Also in this case the remedy may involve a deterioration in polymer quality as well as a decrease in reactor productivity.
Patent EP-B-232701 describes the use of antistatic agents to prevent the formation of crusts inside the reactor during processes for the preparation of ultra high molecular weight polyethylene (UHMWPE) wherein the polymer is in the form of a powder having an average particle diameter smaller than 1 mm and wherein the antistatic agent is used to solve the problems associated with the presence of electrostatic charges in the ultra high molecular weight polyethylene powders. The preferred antistatic agent is a mixture of a chromium organic salt with a calcium organic salt and a phenolic stabilizer, and has to be used in an amount lower than 200 ppm, preferably comprised between 5 and 100 ppm, in order not to interfere with the catalyst activity.
The antistatic agent prevents the formation of crusts inside the reactor but, as clearly shown in subsequent patents EP-A-362629 and EP-A-364759, the polymers have a rather low bulk density and in the films obtained therefrom impurities are present in the form of unmelted products.
These last patents suggest a pretreatment of the catalyst with the antistatic agent, in order to eliminate these drawbacks. To this purpose the antistatic agent, used in an amount of a few ppm by weight with respect to the final polymer but which may reach up to 1,000% by weight with respect to the catalyst, must not contain functional groups capable of deactivating the catalyst. Also by this route a certain amount of impurities still remains in the films obtained from these polymers.
Patent EP-B-229368 describes the use of antistatic agents to prevent the formation of crusts inside the reactor during polymerization or copolymerization processes of ethylene in the gas phase.
The preferred antistatic agent is a mixture of a chromium organic salt with a calcium organic salt and a phenol stabilizer and has to be used in an amount lower than 100 ppm with respect to the polymer in order not to interfere with the catalyst activity.
Other processes for reducing or eliminating the electrostatic voltage include (1) installation of grounding devices in a fluidized bed, (2) ionization of gas or particles by electrical discharge to generate ions which neutralize electrostatic charges on the particles and (3) the use of radioactive sources to produce radiation capable of generating ions which neutralize electrostatic charges on the particles.
However, the use of these techniques in an industrial scale fluid bed polymerization reactor is generally neither practical or easy.
Fluidized or stirred beds consist of polymer particles having a defined geometric shape and a granulometric distribution preferably narrow and generally distributed over values higher than 500 .mu.m.
The presence of a significant amount of fine particles mainly deriving from breakage of a portion of the catalyst gives rise to the problem of the adhesion of these particles to the reactor walls.
None of the techniques proposed to date for preventing adhesion of the polymer to the reactor walls during gas-phase ethylene polymerization processes in fluid bed systems provides a solution to the problem of inhibiting the reactivity of the polymer particle fines, which problem is to be considered among the main causes responsible for the adhesion phenomenon and for the sheeting drawbacks deriving therefrom.
Therefore, the need is felt for solutions which do not decrease the activity of the catalyst system, as it conversely occurs by using chemical compounds inhibiting polymerization reactions, and which at the same time inhibit the polymerization of fine particles which generally leads to the formation of sheets.